Anisotropic Conductive Elastomer (ACE) is a composite of conductive metal elements in an elastomeric matrix that is normally constructed such that it conducts along one axis only. In general, ACE is made to conduct through its thickness. One form of ACE material is made by mixing magnetic particles with a liquid resin, forming the mix into a continuous sheet, and curing the sheet in the presence of a magnetic field. This results in the particles forming a large number of closely spaced columns through the sheet thickness. The columns are electrically conductive. The resulting structure has the unique property of being both flexible and anisotropically conductive. These properties provide for an enabling interconnection medium which, when combined with other technologies, make it possible to realize new interconnect capabilities.
BGA (ball grid array) devices are electronic components with solder balls placed in a grid and used for final installation (through a solder reflow process) on a printed circuit board. There are other types of electronic packages in which the invention is relevant, including Land Grid Arrays (LGA) that have flat-topped pads (lands) with minimal height. Also, Column Grid Arrays (CGA) have protruding columns rather than balls or pads. The needs of the specific application determine which geometry is used.
It is important for the surface of the ball, pad or column that touches the circuit pad to be as free of defects as possible. Deformation of the solder ball of a BGA, especially in the apex area, can result in a defective solder joint. The BGA device is typically tested using a custom socket assembly that utilizes spring-loaded contacts. These contacts may have a serrated face that allows tangential contact with individual solder balls.
Although a serrated contact face provides an excellent electrical contact, the overall length of the pin creates a higher electrical inductance for the contact pin assembly as compared to the short path length provided by the ACE material. As microprocessor speeds increase, inductance hampers performance. Some manufacturers have seen this degradation at bandwidths as low as 500 MHz.
Spring loaded probe pins have long been used by the electronic industry to separably contact electrical systems. FIGS. 4A and 4B show a generic probe pin 69. Such a probe pin generally consists of three elements: a barrel 70, a spring 74, and a plunger 72, that together provide compliant electrical contact between two members (not shown in the drawing). There are many variations on the basic design, and such pins are made by many companies and sold under numerous trade names, such as the PogoPin® made by Everett Charles. As the electronic industry has evolved to higher speed and finer pitch, the required size of the probe pins has become a major manufacturing challenge. Furthermore, the performance and cost of these small probe pins does not meet the electronic industry expectations.